1. Field of the Invention
The present invention relates to a cathode active material comprising a carbon compound and a lithium battery using the same, and more particularly, to a cathode active material that provides a lithium battery with an improved discharging capacity and cycle life characteristics and ensures safety by contributing to cutting off the flow of a current when the battery is overcharged, and a lithium battery using the cathode active material.
2. Description of the Related Art
As portable electronic devices, such as camcorders, mobile phones, notebook computers, and the like, become smaller and more lightweight while at the same time becoming functionally more powerful, considerable research has been conducted into batteries as a driving source for these portable devices. In particular, rechargeable lithium secondary batteries have three times greater energy densities per unit weight and higher charging rates relative to conventional lead batteries, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen batteries, nickel-zinc batteries, and the like. For these reasons, lithium secondary batteries have received greater attention, and intensive research thereinto has been conducted.
Examples of cathode active materials that are widely used in lithium secondary batteries include lithium composite oxides, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4, LiNixCo1-xO2, LiNi1-x-yCoxMnyO2, and the like. Examples of anode active materials that are commonly used include lithium metal, lithium metal alloys, carbonaceous materials, graphitic materials, and the like.
In addition, lithium secondary batteries include a separator to insulate a cathode and an anode, and an electrolyte acting as a lithium ion transfer medium. A carbonate-based organic solvent is widely used as the electrolyte.
Lithium secondary batteries include a cathode, an anode, a separator, and an electrolyte and are sealed with a container, such as a stainless steel can, an aluminum pouch, and the like. Accordingly, when a lithium secondary battery is overcharged due to the overflow of a current caused by abnormal operation, the electrolyte in the lithium secondary battery decomposes and forms gases, raising the internal pressure of the battery and breaking the container.
For this reason, a current cut-off device is incorporated into lithium secondary batteries. For example, the current cut-off device operates when the internal pressure of a battery rises excessively and separates a cathode tap from a cathode current collector to cut off the flow of current. However, this current cut-off device operates depending on the internal pressure of a battery, i.e., only when the internal pressure of a battery rises above a predetermined level. If there is no significant rise in internal pressure, the current cut-off device may not operate even when the temperature rises above a normal range.
To address this problem, U.S. Pat. No. 5,427,875 discloses the addition of 0.5-15% by weight of lithium carbonate (Li2CO3) in the manufacture of a cathode to allow a current cut-off device to operate in response to a rise in the internal temperature of a battery, wherein the lithium carbonate in the cathode generates a carbonic acid gas when the internal temperature of the battery rises, raising the internal pressure and initiating the operation of the current cut-off device. In Japanese Laid-open Patent No. 2003-151155, a cathode made of a mixture of a cathode active material containing 0.5% or less by weight of lithium carbonate and a cathode active material containing 0.5-5% by weight of lithium carbonate is utilized to allow a current cut-off device to operate more promptly in response to a rise in internal pressure and to improve a performance of the battery, such as discharging capacity and cycle life.
However, the above patents require an additional step of adding lithium carbonate to a cathode active material. In addition, it is difficult to obtain a uniform dispersion of lithium carbonate with the methods disclosed in the above patents.